The present invention relates to drilling equipment for drilling holes in a workpiece such as a circuit board, and more particularly to a drilling spindle for feeding drilling tools having a relatively small diameter drilling bit into such a workpiece employing a pressure foot to clamp the workpiece and yet allow the drilling tool to be changed during a tool changing operation.
Printed circuit boards are virtually universal use today to mount and interconnect electrical components forming electrical circuits. Typically, the leads of the components are inserted through holes drilled in the boards to form predetermined hole patterns. Various systems are known in the art for automated drilling of the holes in printed circuit boards. Examples of such systems and components thereof are described in U.S. Pat. Nos. 4,088,417; 3,578,827 and 3,973,863. A high speed drilling system is described in U.S. Pat. No. 4,761,876 issued Aug. 9, 1988, for a High Speed Precision Drilling System, assigned to a common assignee with the present application, the entire contents of which are incorporated herein by this reference.
In order to securely clamp the workpiece during the drilling operation, a spindle pressure foot is conventionally employed, as described in U.S. Pat. No. 4,088,417. The pressure foot is mounted for translation along the spindle rotor axis and includes an aperture formed therein for permitting the drilling tool to be passed therethrough for drilling and tool changing operations. Means are typically provided for lowering the pressure foot to contact the workpiece during drilling of the workpiece, and for raising the pressure foot after completion of the drilling operation. Such pressure foot apparatus performs satisfactorily for drilling operations which employ drilling tools or bits having a uniform diameter through its shank and drilling bit portion. With these types of drilling tools, the conventional pressure foot may be employed satisfactorily to insert and remove the drilling tool from the drilling tool during tool changing operations. The aperture in the pressure foot through which the drilling tool is fed through during the drilling operation also allows the entire tool to be inserted and removed during the changing operation. A typical size for the aperture in a conventional pressure foot is 0.8 inches.
In U.S. Pat. No. 4,761,876, a new design of drilling tool is disclosed for drilling small diameter holes in the workpiece, and is particularly adapted to use with the new clip-fed tool changer disclosed in that application. This new drilling tool is characterized by a relatively large diameter shank portion which narrows to a small diameters drill tip, with a groove formed in the shank for fitting with the tool changer capture plate. Alternatively, a collar is fitted on the shank of the drill tool with a groove formed therein formed to serve a similar purpose. One problem with using a conventional pressure foot with such a drilling tool is that the aperture in the pressure foot must be made large enough to accommodate the relatively large diameter shank or collar so that the tool may be changed through the opening in the pressure foot. Because the drill bit portion is substantially smaller than the shank, e.g., less than 0.050 inches, the clamping of the workpiece by the pressure foot is distanced from the bit, allowing the workpiece to climb up the bit as the bit is fed into the workpiece. This leads to drill "wander" and decreases the quality of the hole.
It would therefore represent an advance in the art to provide a drilling spindle with a pressure foot apparatus which securely clamps the workpiece during drilling operations, yet allows the drilling tools to be changed conveniently.